Optical phase-sensitive amplifier with fiber bragg grating phase shifter

ABSTRACT

Fiber Bragg gratings (FBG) may be used to perform phase adjustment for optimal phase-sensitive amplification. Specifically, FBGs may be used between the idler stage and the amplification stage of an optical phase-sensitive amplifier for phase shifting or tuning. The phase shifting or tuning may be applied to at least one of an input optical signal, an idler signal, and an optical pump. A feedback control loop may be used in the phase-sensitive optical amplifier with respect to an output optical signal for optimal phase adjustment.

BACKGROUND Field of the Disclosure

The present disclosure relates generally to optical communicationnetworks and, more particularly, to an optical phase-sensitive amplifierwith fiber Bragg grating phase shifter.

Description of the Related Art

Telecommunication, cable television and data communication systems useoptical networks to rapidly convey large amounts of information betweenremote points. In an optical network, information is conveyed in theform of optical signals through optical fibers. Optical fibers maycomprise thin strands of glass capable of communicating the signals overlong distances. Optical networks often employ modulation schemes toconvey information in the optical signals over the optical fibers. Suchmodulation schemes may include phase-shift keying (PSK), frequency-shiftkeying (FSK), amplitude-shift keying (ASK), and quadrature amplitudemodulation (QAM).

Optical networks may also include various optical elements, such asamplifiers, dispersion compensators, multiplexer/demultiplexer filters,wavelength selective switches (WSS), optical switches, couplers, etc. toperform various operations within the network. In particular, opticalnetworks may include costly optical-electrical-optical (O-E-O)regeneration at colorless, directionless, contentionless reconfigurableoptical add-drop multiplexers (CDC ROADMs) when the reach of an opticalsignal is limited in a single optical path.

As data rates for optical networks continue to increase, reaching up to1 terabit/s (1 T) and beyond, the demands on optical signal-to-noiseratios (OSNR) also increase, for example, due to the use of advancedmodulation formats, such as QAM and PSK with dual polarization. Inparticular, noise accumulations resulting from cascading of opticalamplifiers in an optical network operating at very high data rates maylimit the reach of an optical signal at a desired level of OSNR and mayresult in an increased number of O-E-O regenerations, which iseconomically disadvantageous.

SUMMARY

In one aspect, an optical system for phase-sensitive amplification ofoptical signals is disclosed. The optical system may include an inputoptical signal and a phase-sensitive amplifier (PSA) stage I receivingthe input optical signal. In the optical system, the PSA stage I mayinclude a first non-linear optical element (NLE) through which the inputoptical signal and a first pump wavelength are passed to generate a PSAstage I optical signal comprising the input optical signal, the firstpump wavelength, and an idler signal generated using the first NLE. Theoptical system may further include a fiber Bragg grating (FBG) receivingthe PSA stage I optical signal. In the optical system, the FBG mayinclude a plurality of FBG elements, further including a first FBGelement configured to apply a first phase shift to at least one of theinput optical signal, the first pump wavelength, and the idler signal.In the optical system, the FBG may output the PSA stage I optical signalwith the first phase shift as a PSA stage II optical signal, while theinput optical signal, the first pump wavelength, and the idler signalmay be phase-matched in the PSA stage II optical signal.

In any of the disclosed embodiments, the optical system may furtherinclude a PSA stage II receiving the PSA stage II optical signal. In theoptical system, the PSA stage II may include a second NLE through whichthe PSA stage II optical signal is amplified to generate an outputoptical signal.

In any of the disclosed embodiments, the PSA stage II may furtherinclude a Raman amplifier, and a second pump wavelength for transmissionthrough the Raman amplifier in a counter propagating direction to thePSA stage II signal.

In any of the disclosed embodiments of the optical system, the inputoptical signal may include one optical channel, while the first FBGelement may apply the first phase shift to the first pump wavelength.

In any of the disclosed embodiments of the optical system, the inputoptical signal may include a wavelength division multiplexed (WDM)optical signal, further including a first optical channel, while thefirst FBG element may apply the first phase shift to a first idlersignal that is a conjugate of the first optical channel.

In any of the disclosed embodiments, the optical system may furtherinclude a first heating element associated with the first FBG element,where the first heating element may be used to control a localtemperature of the first FBG element to control the first phase shift.

In any of the disclosed embodiments, the optical system may furtherinclude a plurality of heating elements respectively corresponding tothe FBG elements, where each of the FBG elements may apply a respectivephase shift to the PSA stage I optical signal.

In any of the disclosed embodiments, the optical system may furtherinclude a first tension element associated with the first FBG element,where the first tension element may be used to control a local strain ofthe first FBG element to control the first phase shift.

In any of the disclosed embodiments, the optical system may furtherinclude a plurality of tension elements respectively corresponding tothe FBG elements, where each of the FBG elements applies a respectivephase shift to the PSA stage I optical signal.

In any of the disclosed embodiments, the optical system may furtherinclude a feedback control loop from the PSA stage II to the FBG, wherethe feedback control loop is used to spectrally align the first phaseshift.

Other disclosed aspects include an optical phase-sensitive amplifierwith FBG phase shifter, as well as an optical network including opticalphase-sensitive amplifiers with FBG phase shifters.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram of selected elements of an embodiment of anoptical network;

FIG. 2A depicts plots of example optical properties of fiber Bragggratings (FBG);

FIG. 2B is a block diagram of selected elements of an embodiment of anoptical phase-sensitive amplifier with FBG phase shifter;

FIG. 3 is a block diagram of selected elements of an embodiment of aphase-sensitive optical amplifier stage I;

FIG. 4 is a block diagram of selected elements of an embodiment of aphase-sensitive optical amplifier stage II; and

FIG. 5 is a diagram of selected elements of an embodiment of an FBGphase shifter.

DESCRIPTION OF PARTICULAR EMBODIMENT(S)

In the following description, details are set forth by way of example tofacilitate discussion of the disclosed subject matter. It should beapparent to a person of ordinary skill in the field, however, that thedisclosed embodiments are exemplary and not exhaustive of all possibleembodiments.

Throughout this disclosure, a hyphenated form of a reference numeralrefers to a specific instance of an element and the un-hyphenated formof the reference numeral refers to the element generically orcollectively. Thus, as an example (not shown in the drawings), device“12-1” refers to an instance of a device class, which may be referred tocollectively as devices “12” and any one of which may be referred togenerically as a device “12”. In the figures and the description, likenumerals are intended to represent like elements.

Referring now to the drawings, FIG. 1 illustrates an example embodimentof optical network 101, which may represent an optical communicationsystem. Optical network 101 may include one or more optical fibers 106to transport one or more optical signals communicated by components ofoptical network 101. The network elements of optical network 101,coupled together by fibers 106, may comprise one or more transmitters102, one or more multiplexers (MUX) 104, one or more optical amplifiers108, one or more optical add/drop multiplexers (OADM) 110, one or moredemultiplexers (DEMUX) 105, and one or more receivers 112.

Optical network 101 may comprise a point-to-point optical network withterminal nodes, a ring optical network, a mesh optical network, or anyother suitable optical network or combination of optical networks.Optical network 101 may be used in a short-haul metropolitan network, along-haul inter-city network, or any other suitable network orcombination of networks. The capacity of optical network 101 mayinclude, for example, 100 Gbit/s, 400 Gbit/s, or 1 Tbit/s. Opticalfibers 106 comprise thin strands of glass capable of communicating thesignals over long distances with very low loss. Optical fibers 106 maycomprise a suitable type of fiber selected from a variety of differentfibers for optical transmission. Optical fibers 106 may include anysuitable type of fiber, such as a Single-Mode Fiber (SMF), EnhancedLarge Effective Area Fiber (E-LEAF), or TrueWave® Reduced Slope (TW-RS)fiber.

Optical network 101 may include devices to transmit optical signals overoptical fibers 106. Information may be transmitted and received throughoptical network 101 by modulation of one or more wavelengths of light toencode the information on the wavelength. In optical networking, awavelength of light may also be referred to as a channel that isincluded in an optical signal. Each channel may carry a certain amountof information through optical network 101.

To increase the information capacity and transport capabilities ofoptical network 101, multiple signals transmitted at multiple channelsmay be combined into a single wideband optical signal. The process ofcommunicating information at multiple channels is referred to in opticsas wavelength division multiplexing (WDM). Coarse wavelength divisionmultiplexing (CWDM) refers to the multiplexing of wavelengths that arewidely spaced having low number of channels, usually greater than 20 nmand less than sixteen wavelengths, and dense wavelength divisionmultiplexing (DWDM) refers to the multiplexing of wavelengths that areclosely spaced having large number of channels, usually less than 0.8 nmspacing and greater than forty wavelengths, into a fiber. WDM or othermulti-wavelength multiplexing transmission techniques are employed inoptical networks to increase the aggregate bandwidth per optical fiber.Without WDM, the bandwidth in optical networks may be limited to thebit-rate of solely one wavelength. With more bandwidth, optical networksare capable of transmitting greater amounts of information. Opticalnetwork 101 may transmit disparate channels using WDM or some othersuitable multi-channel multiplexing technique, and to amplify themulti-channel signal.

Optical network 101 may include one or more optical transmitters (Tx)102 to transmit optical signals through optical network 101 in specificwavelengths or channels. Transmitters 102 may comprise a system,apparatus or device to convert an electrical signal into an opticalsignal and transmit the optical signal. For example, transmitters 102may each comprise a laser and a modulator to receive electrical signalsand modulate the information contained in the electrical signals onto abeam of light produced by the laser at a particular wavelength, andtransmit the beam for carrying the signal throughout optical network101.

Multiplexer 104 may be coupled to transmitters 102 and may be a system,apparatus or device to combine the signals transmitted by transmitters102, e.g., at respective individual wavelengths, into a WDM signal.

Optical amplifiers 108 may amplify the multi-channeled signals withinoptical network 101. Optical amplifiers 108 may be positioned before orafter certain lengths of fiber 106. Optical amplifiers 108 may comprisea system, apparatus, or device to amplify optical signals. For example,optical amplifiers 108 may comprise an optical repeater that amplifiesthe optical signal. This amplification may be performed withopto-electrical or electro-optical conversion. In some embodiments,optical amplifiers 108 may comprise an optical fiber doped with arare-earth element to form a doped fiber amplification element. When asignal passes through the fiber, external energy may be applied in theform of an optical pump to excite the atoms of the doped portion of theoptical fiber, which increases the intensity of the optical signal. Asan example, optical amplifiers 108 may comprise an erbium-doped fiberamplifier (EDFA).

OADMs 110 may be coupled to optical network 101 via fibers 106. OADMs110 comprise an add/drop module, which may include a system, apparatusor device to add and drop optical signals (for example at individualwavelengths) from fibers 106. After passing through an OADM 110, anoptical signal may travel along fibers 106 directly to a destination, orthe signal may be passed through one or more additional OADMs 110 andoptical amplifiers 108 before reaching a destination.

In certain embodiments of optical network 101, OADM 110 may represent areconfigurable OADM (ROADM) that is capable of adding or droppingindividual or multiple wavelengths of a WDM signal. The individual ormultiple wavelengths may be added or dropped in the optical domain, forexample, using a wavelength selective switch (WSS) (see also FIG. 2)that may be included in a ROADM. ROADMs are considered ‘colorless’ whenthe ROADM is able to add/drop any arbitrary wavelength. ROADMs areconsidered ‘directionless’ when the ROADM is able to add/drop anywavelength regardless of the direction of propagation. ROADMs areconsidered ‘contentionless’ when the ROADM is able to switch anycontended wavelength (already occupied wavelength) to any otherwavelength that is available.

As shown in FIG. 1, optical network 101 may also include one or moredemultiplexers 105 at one or more destinations of network 101.Demultiplexer 105 may comprise a system apparatus or device that acts asa demultiplexer by splitting a single composite WDM signal intoindividual channels at respective wavelengths. For example, opticalnetwork 101 may transmit and carry a forty (40) channel DWDM signal.Demultiplexer 105 may divide the single, forty channel DWDM signal intoforty separate signals according to the forty different channels.

In FIG. 1, optical network 101 may also include receivers 112 coupled todemultiplexer 105. Each receiver 112 may receive optical signalstransmitted at a particular wavelength or channel, and may process theoptical signals to obtain (e.g., demodulate) the information (i.e.,data) that the optical signals contain. Accordingly, network 101 mayinclude at least one receiver 112 for every channel of the network.

Optical networks, such as optical network 101 in FIG. 1, may employmodulation techniques to convey information in the optical signals overthe optical fibers. Such modulation schemes may include phase-shiftkeying (PSK), frequency-shift keying (FSK), amplitude-shift keying(ASK), and quadrature amplitude modulation (QAM), among other examplesof modulation techniques. In PSK, the information carried by the opticalsignal may be conveyed by modulating the phase of a reference signal,also known as a carrier wave, or simply, a carrier. The information maybe conveyed by modulating the phase of the signal itself using two-levelor binary phase-shift keying (BPSK), four-level or quadraturephase-shift keying (QPSK), multi-level phase-shift keying (M-PSK) anddifferential phase-shift keying (DPSK). In QAM, the information carriedby the optical signal may be conveyed by modulating both the amplitudeand phase of the carrier wave. PSK may be considered a subset of QAM,wherein the amplitude of the carrier waves is maintained as a constant.

Additionally, polarization division multiplexing (PDM) technology mayenable achieving a greater bit rate for information transmission. PDMtransmission comprises independently modulating information ontodifferent polarization components of an optical signal associated with achannel. In this manner, each polarization component may carry aseparate signal simultaneously with other polarization components,thereby enabling the bit rate to be increased according to the number ofindividual polarization components. The polarization of an opticalsignal may refer to the direction of the oscillations of the opticalsignal. The term “polarization” may generally refer to the path tracedout by the tip of the electric field vector at a point in space, whichis perpendicular to the propagation direction of the optical signal.

In an optical network, such as optical network 101 in FIG. 1, it istypical to refer to a management plane, a control plane, and a transportplane (sometimes called the physical layer). A central management host(not shown) may reside in the management plane and may configure andsupervise the components of the control plane. The management planeincludes ultimate control over all transport plane and control planeentities (e.g., network elements). As an example, the management planemay consist of a central processing center (e.g., the central managementhost), including one or more processing resources, data storagecomponents, etc. The management plane may be in electrical communicationwith the elements of the control plane and may also be in electricalcommunication with one or more network elements of the transport plane.The management plane may perform management functions for an overallsystem and provide coordination between network elements, the controlplane, and the transport plane. As examples, the management plane mayinclude an element management system (EMS) which handles one or morenetwork elements from the perspective of the elements, a networkmanagement system (NMS) which handles many devices from the perspectiveof the network, and an operational support system (OSS) which handlesnetwork-wide operations.

Modifications, additions or omissions may be made to optical network 101without departing from the scope of the disclosure. For example, opticalnetwork 101 may include more or fewer elements than those depicted inFIG. 1. Also, as mentioned above, although depicted as a point-to-pointnetwork, optical network 101 may comprise any suitable network topologyfor transmitting optical signals such as a ring, a mesh, and ahierarchical network topology.

As discussed above, the amount of information that may be transmittedover an optical network may vary with the number of optical channelscoded with information and multiplexed into one signal. Accordingly, anoptical fiber employing a WDM signal may carry more information than anoptical fiber that carries information over a single channel. Besidesthe number of channels and number of polarization components carried,another factor that affects how much information can be transmitted overan optical network may be the bit rate of transmission. The higher thebit rate, the greater the transmitted information capacity. Achievinghigher bit rates may be limited by the availability of wide bandwidthelectrical driver technology, digital signal processor technology andincrease in the required OSNR for transmission over optical network 101.

In operation of optical network 101, as data rates approach 1 T andbeyond, a correspondingly high OSNR becomes desirable to maintaineconomic feasibility by avoiding excessive numbers of O-E-Oregenerators. One source of OSNR reduction is the noise accumulationresulting from cascaded optical amplifiers 108 at various points in thetransmission path. For an optical amplifier, OSNR may be represented asa noise figure (NF), given by Equation 1, where OSNR_(in) is the inputOSNR, OSNR_(out) is the output OSNR, and dB is decibels.NF=10 log(OSNR_(in)/OSNR_(out))=OSNR_(in) [dB]−OSNR_(out) [dB]  Equation(1)

Current designs for optical amplifiers may include opticalphase-sensitive amplifiers (PSA), which may exhibit a low noise figure,such as less than 3 dB in many instances. Some PSA designs, such asusing fiber Bragg grating phase shifters as disclosed herein, mayexhibit a lower noise figure than 3 dB noise figure, such as less than 2dB noise figure, or less than 1 dB noise figure. The lower noise figuremay enable an increased optical reach for a given optical signal, whichis desirable.

A typical phase-sensitive optical amplifier will have different stages,including an idler stage to initially generate an idler signal using anoptical pump and an amplification stage to amplify the input signalusing the optical pump and the idler signal. In between the idler stageand the amplification stage, an intermediate stage may be implemented inthe phase-sensitive optical amplifier. The intermediate stage mayinvolve complex signal processing and pump power recovery to adjust thepower level of the input signal and the idler signal. In typicalphase-sensitive optical amplifiers, the optical paths of the inputsignal, the optical pump, and the idler signal may be separated in theintermediate stage in order to independently modulate power of each ofthe signals.

When the separated optical paths are recombined prior to theamplification stage, a phase adjustment may be performed to re-align thephase of the signals. In some embodiments, the phase adjustment mayinvolve an optical phase lock loop to re-align the phases of the inputsignal and the idler signal with the optical pump. However, a phase lockloop may not provide the desired stability and robust operation that isdesired for optical networking, for example, when synchronization islost and must be reset.

Fiber Bragg gratings (FBG) are a type of specialized reflector that maybe constructed using periodic variations in an optical fiber. The FBGmay be constructed to reflect a relatively narrow wavelength band, andis also used as a notch filter, accordingly. Thus, FBGs may haverelatively wide transmission bands that allow for transmission ofcertain bandwidths, such as the optical signals described above. Inaddition, FBGs may provide a certain degree of phase shift in thetransmission band. The phase shift provided by an FBG may further betunable, for example, by controlling the temperature of the opticalfiber, or by applying a certain amount of strain to the optical fiber.

As will be described in further detail, methods and systems aredisclosed herein for implementing a phase-sensitive amplifier (PSA) withFBG phase shifter. The PSA with FBG phase shifter disclosed herein maybe implemented using an FBG with multiple FBG elements, where each FBGelement corresponds to a particular channel or group of channels. Thenusing either temperature or strain control of the optical fiber, the PSAwith FBG phase shifter disclosed herein may enable phase matching ofoptical signals and pump signals. The PSA with FBG phase shifterdisclosed herein may be used to provide a PSA having 0 dB noise figure,which may, in turn, enable extension of the reach of optical signalstransmitted over optical networks.

FIG. 2A depicts plots of example optical properties of fiber Bragggratings (FBGs). Specifically, a plot 200 shows transmittance versuswavelength for an FBG. Plot 200 is a schematic illustration and does notshow actual measured data. However, plot 200 illustrates the notchfilter characteristics of an FBG, in which the transmittance is reducedfor a relatively narrow wavelength band. Also depicted in plot 200 is anexample of a transmittance band 220 that may be utilized for the PSAwith FBG phase shifter disclosed herein. As shown, transmittance band220 is located in a region having close to 0 dB (100%) transmittance,but adjacent to the notch filter reflectance band. When wavelengthscorresponding to transmittance band 220 are passed through an FBG, thewavelengths will be transmitted with relatively small amounts ofattenuation of optical power. In other words, the FBG may appeartransparent for wavelengths in transmittance band 220. Because theparticular construction of the FBG may be selected such thattransmittance band 220 falls along a desired wavelength subrange,multiple FBGs may be constructed in a sequential manner (see also FIG.5) along an optical fiber for use with a desired set of optical signals,such as a WDM signal.

Also shown in FIG. 2A is plot 201, which shows phase shift for the sameFBG referred to in plot 200, over the same range of wavelengths.Specifically, transmittance band 220 in plot 200 corresponds to the samewavelength range as phase shift band 222. In phase shift band 222, adiscrete amount of phase shift may be added. Also, for other wavelengthssmaller than in phase shift band 222, the phase shift is zero orsubstantially zero, while the transmittance is close to 0 dB, whichallows for pass through of other frequencies without having a phaseshift applied by the FBG. Furthermore, the value of the phase shift inplot 201 may be varied slightly in wavelength by applying temperature orstrain to the fiber, which can also be used to tune the amount of phaseshift applied to a given wavelength within phase shift band 222. Becausethe actual phase shift applied at phase shift band 222 may be tuned,such as by using temperature or strain control of the FBG, the FBG maybe used for phase shifting or phase adjustment in a PSA.

In summary, the optical properties shown in FIG. 2A describe how a FBGmay be used in an optical fiber for phase shifting. The construction ofthe FBG, for example with multiple individual FBG elements, maydetermine the frequency subband of transmittance band 220 and phaseshift band 222. Then, temperature or strain control at an individual FBGelement may be used to tune the amount of phase shift at the individualFBG element. In this manner, the FBG may be used for phase shifting inan optical PSA, as described in further detail below.

FIG. 2B is a block diagram of selected elements of an embodiment of anoptical PSA with FBG phase shifter 202. In PSA 202, a WDM optical signal210 may be received by a PSA stage I 204. In PSA stage I 204, simplefour wave mixing (FWM) may occur to generate so-called “idler signals”,which are conjugate wavelengths of an optical signal, such as WDMoptical signal 210, relative to a pump wavelength. In FWM, the idlersignals appear when the optical signal and the pump wavelength arepassed through a non-linear element (NLE), such as a highly non-linearfiber (HNLF). In various embodiments, other NLEs may also be used tofacilitate FWM, such as optical crystals or other optical materials. Inthe NLE, photons are converted from the pump wavelength and the opticalsignal to the idler signal by non-linear processes.

Accordingly, PSA stage I outputs a PSA stage I optical signal 230, inwhich the intensity of the pump wavelength and the optical signal isdiminished, but in which the idler signals have been added. Conjugateidler signals may appear in PSA stage I optical signal 230 for eachchannel in the WDM optical signal 210. It is noted that PSA stage I 204may be used with an input signal that includes a single optical channel.

Then, in PSA with FBG phase shifter 202, PSA stage I optical signal 230may be received by FBG phase shifter 208, which is described in furtherdetail below with respect to FIG. 5. In FBG phase shifter 208, the phaseof at least one of the pump wavelength, WDM optical signal 210, and theidler signals may be adjusted. The phase shift applied by FBG phaseshifter 208 may be controlled using a feedback loop 218 that enablesmonitoring of output WDM output signal 214 for precise phase alignment.For example, power variations in output WDM output signal 214 may beobserved to be minimized when the output of FBG phase shifter 208, a PSAstage II optical signal 232, is precisely phase-matched with respect tothe WDM optical signal 210, the pump wavelength, and the idler signals.

Finally, in PSA with FBG phase shifter 202, a PSA stage II 206 mayreceive PSA stage II optical signal 232 and may amplify WDM opticalsignal 210. PSA stage II 206 may also include Raman amplification, aswell as other elements described in further detail below, in order togenerate output WDM output signal 214, in which the channels have beenamplified relative to input WDM optical signal 210.

Referring now to FIG. 3, selected elements of an embodiment of anoptical PSA stage I 204 are depicted. In FIG. 3, optical PSA stage I 204is shown in a schematic representation and is not drawn to scale. It isnoted that, in different embodiments, optical PSA stage I 204 may beoperated with additional or fewer elements as shown in FIG. 3.

In FIG. 3, optical PSA stage I 204 receives WDM input signal 210 andadds optical pump 308 using coupler 306. Intermediate stage I signal312, comprising WDM input signal 210 and optical pump 308 are then sentto NLE idler 314, which is a non-linear optical element. In the presenceof optical pump 308 and WDM input signal 210, simple four wave mixing(FWM) may occur at NLE idler 314 to generate idler signals 318,resulting in PSA stage I optical signal 230, as described above withrespect to FIG. 2B.

Also shown in FIG. 3 are spectra of the different signals transmitted inoptical PSA stage I 204. In spectra 210-S, optical signal 310 representsone or more wavelengths included in WDM input signal 210. In spectra312-S, corresponding to intermediate stage I signal 312, optical pump308 is added to optical signal 310. In spectra 230-S corresponding toPSA stage I optical signal 230, idler signal 318 has been added,representing corresponding one or more wavelengths of optical signal310, but spectrally spaced symmetrically with respect to optical pump308. Also, the optical power of the signals in spectra 230-S has beenreduced, which is indicative of FWM to generate idler signal 318.

Referring now to FIG. 4, selected elements of an embodiment of anoptical PSA stage II 206 are depicted. In FIG. 4, optical PSA stage II206 is shown in a schematic representation and is not drawn to scale. Itis noted that, in different embodiments, optical PSA stage II 206 may beoperated with additional or fewer elements as shown in FIG. 4.

In FIG. 4, optical PSA stage II 206 receives PSA stage II optical signal232 from FBG phase shifter 208. Accordingly, the signals in PSA stage IIoptical signal 232 may be phase matched by FBG phase shifter, asdisclosed herein. PSA stage II optical signal 232 may be passed throughisolator 402 to prevent back propagation of Raman pump 424, beforesending PSA stage II optical signal 232 to NLE Raman amplification 422,which receives Raman pump 424 using coupler 406 in a counter propagatingdirection. PSA stage II optical signal 232 may include optical signal310, which comprises the wavelengths in input WDM optical signal 210, asdescribed above, along with corresponding idler signals 318 and opticalpump 308. As noted, optical signals shown in spectra 232-S may be phasematched, or phase aligned, using FBG phase shifter 208 for optimaloperation of PSA stage II 206.

In optical PSA stage II 206, NLE Raman amplification 422 may comprise aRaman amplifier that includes Raman pump 424, which may be a lasersource, that is directed through an NLE as a gain medium in a counterpropagation direction to the optical signal being processed (PSA stageII optical signal 232). Raman pump 424 may be selected based on the gainmedium used. For example, a 13 THz optical pump may be used withGeO₂/SiO₂ single mode fibers (SMF) as the NLE, while a 40 THz opticalpump may be used with P₂O₅-doped SiO₂ phosphate-doped fiber (PDF) as theNLE in NLE Raman amplification 422. Furthermore, modulation ormodification of the optical power of Raman pump 424 may be used todetermine or modify an optical gain of NLE Raman amplification 422. Itis noted that Raman amplification may be optional in some embodiments ofoptical PSA stage II 206, such that isolator 402, NLE Ramanamplification 422, coupler 406, and Raman pump 424 may be omitted (notshown).

The output of NLE Raman amplification 422 is shown as Raman amplifiedoptical signal 412, which is directed to NLE amplification 418, which isa non-linear optical element. In the presence of Raman amplified opticalsignal 412, one-pump four wave mixing (FWM) may occur at NLEamplification 418 to amplify the WDM optical signal and the idlersignals, at the expense of optical pump 308. NLE amplification 418 mayinclude components for performing one-pump optical four-wave mixing(FWM). The one-pump FWM may be accomplished by passing the input signal,or filtered portions thereof, through a non-linear optical element(NLE), such as a doped optical fiber, periodically poled lithium niobate(PPLN), aluminium gallium arsenide (AlGaAs) or other semiconductormaterial that exhibits desired optical non-linearity.

After NLE amplification 418, optical signal 414 includes the amplifiedWDM optical signals and idler signals, along with the diminished opticalpump 308. An optical bandpass filter (OBPF) 408 may then be applied toisolate WDM output signal 214.

Also shown in FIG. 4 are spectra of the different signals transmitted inoptical PSA stage II 206. In spectra 232-S, optical signal 310represents one or more wavelengths included in WDM output signal 214,while idler signals 318 are conjugates of optical signal 310 withrespect to optical pump 308. As noted, optical signals shown in spectra232-S may be phase matched, or phase aligned, using FBG phase shifter208 for optimal operation of PSA stage II 206.

In spectra 412-S, corresponding to Raman amplified optical signal 412,optical signal 310, optical pump 308, and idler signals 318 may beamplified (shown with increased signal intensity). In spectra 414-S,optical signal 310 and idler signals 318 may be amplified at the expenseof optical pump 308, corresponding to FWM. In spectra 214-S, opticalsignal 310 is isolated in amplified form to generate WDM output signal214.

FIG. 5 is a diagram of selected elements of an embodiment of an FBGphase shifter 208-1. FIG. 5 is a schematic illustration and is not drawnto scale or perspective. In FIG. 5, FBG phase shifter 208-1 comprises anoptical fiber 502 and is enabled to receive PSA stage I optical signal230 and to output PSA stage II optical signal 232, as describedpreviously. Optical fiber 502 in FBG phase shifter 208-1 is furthercomprised of a cladding 506 and a fiber core 504 that transmits opticalsignals due to differences in the index of refraction within opticalfiber 502.

Within fiber core 504, a plurality of FBG elements 510 are shown,including FBG elements 510-1, 510-2, and 510-3. FBG elements 510 areshown equivalent in FIG. 5 for descriptive clarity, however, eachindividual FBG element 510 may be particularly designed or constructedto transmit a range of wavelengths, for example, corresponding towavelengths in PSA stage I optical signal 230 (and PSA stage II opticalsignal 232), as described previously. In some embodiments, eachindividual channel or wavelength may be phase shifted using anindividual FBG element 510.

Also shown in FIG. 5 are heating devices 512 corresponding to eachindividual FBG element 510. Specifically, heating device 512-1 may beused to individually control temperature of FBG element 510-1; heatingdevice 512-2 may be used to individually control temperature of FBGelement 510-2; and heating device 512-3 may be used to individuallycontrol temperature of FBG element 510-3. Heating devices 512 may alsoinclude a temperature sensor and control logic (not shown), such as amicrocontroller or microprocessor having access to a non-transitorymemory media storing executable instructions to perform temperaturecontrol for a corresponding FBG element 510. Accordingly, each FBGelement 510/heating device 512 pair may operate independently withregard to temperature control, in order to provide individual phaseshifting, such as within phase shift band 222 (see FIG. 2A). Because thephase shift of each FBG element 510 is dependent on temperature, in thismanner, the phase shift applied to each FBG element 510 may becontrolled as desired.

In other embodiments, heating device 512 may be replaced, or augmented,with a strain control device (not shown), such as a tension element(i.e., a tensioner) that applies a defined tension to optical fiber 502at a location corresponding to the location of FBG element 510 alongoptical fiber 502. Because the phase shift of each FBG element is alsodependent on strain in fiber core 504, the phase shift applied to eachFBG element 510 may be controlled as desired using the tension elements.In some embodiments, a combination of temperature and strain control ofFBG elements 510 may be used.

In one example, operation of FBG phase shifter 208-1 may includeadjusting the phase of idler signals 318. For example, it may be assumedthat FBG phase shifter 208-1 receives, as input, signals and wavelengthscorresponding to PSA stage I optical signal 230 having 3 channels andcorresponding to spectra 230-S (see FIG. 3). Accordingly, opticalsignals 310 and idler signals 318 have 3 channels each that arespectrally symmetric about optical pump 308, and which can bepre-calculated when the wavelengths of the optical pump 308 and opticalsignals 310 are known. In order to apply FBG phase shifter 208-1 to the3 idler signals 318, each individual FBG element 510 may be formed to acorresponding wavelength of the individual idler signal 318. Forexample, FBG element 510-1 may be formed such that its phase shift band222 corresponds to a first idler signal 318, FBG element 510-2 may beformed such that its phase shift band 222 corresponds to a second idlersignal 318, and FBG element 510-3 may be formed such that its phaseshift band 222 corresponds to a third idler signal 318. In this manner,FBG elements 510 may be formed to be selective to individual ones of theidler signals 318, while being transparent to the other idler signals.Then, after optical fiber 502 with the 3 FBG elements 510 has beenformed and the wavelengths corresponding to spectra 230-S aretransmitted through optical fiber 502, a base calibration of heatingelements 512 may be performed, such that each heating element 512 is ata temperature that corresponds to a known output of FBG elements 510.The output of FBG phase shifter 208-1 may then be routed through PSAstage II 206 to generate WDM output signals 214. Then, an output of PSAwith FBG phase shifter 202, namely WDM output signal 214, may bemonitored for optical power, while the phase of each FBG element 510 istuned using heating elements 512. For example, a temperature change ofabout 40° C. may result in a phase shift of about 1 nm in wavelength atFBG element 510. Heating elements 512 may be tuned to respectivetemperatures such that an optical power of WDM output signal 214 ismaximized, which will occur when all phases in idler signals 318 arealigned.

As disclosed herein, fiber Bragg gratings (FBG) may be used to performphase adjustment for optimal phase-sensitive amplification.Specifically, FBGs may be used between the idler stage and theamplification stage of an optical phase-sensitive amplifier for phaseshifting or tuning. The phase shifting or tuning may be applied to atleast one of an input optical signal, an idler signal, and an opticalpump. A feedback control loop may be used in the phase-sensitive opticalamplifier with respect to an output optical signal for optimal phaseadjustment.

The above disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments which fall within thetrue spirit and scope of the present disclosure. Thus, to the maximumextent allowed by law, the scope of the present disclosure is to bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

What is claimed is:
 1. An optical system for phase-sensitiveamplification of optical signals, the optical system comprising: aninput optical signal; a phase-sensitive amplifier (PSA) stage Ireceiving the input optical signal, wherein the PSA stage I comprises afirst non-linear optical element (NLE) through which the input opticalsignal and a first pump wavelength are passed to generate a PSA stage Ioptical signal comprising the input optical signal, the first pumpwavelength, and an idler signal generated using the first NLE; a fiberBragg grating (FBG) receiving the PSA stage I optical signal, whereinthe FBG includes a plurality of FBG elements, including a first FBGelement configured to apply a first phase shift to at least one of theinput optical signal, the first pump wavelength, and the idler signal,wherein the FBG outputs the PSA stage I optical signal with the firstphase shift as a PSA stage II optical signal, and wherein the inputoptical signal, the first pump wavelength, and the idler signal arephase-matched in the PSA stage II optical signal; and a feedback controlloop from the PSA stage II to the FBG, wherein the feedback control loopis used to spectrally align the first phase shift.
 2. The optical systemof claim 1, further comprising: a PSA stage II receiving the PSA stageII optical signal, wherein the PSA stage II comprises a second NLEthrough which the PSA stage II optical signal is amplified to generatean output optical signal.
 3. The optical system of claim 2, wherein thePSA stage II further comprises: a Raman amplifier; and a second pumpwavelength for transmission through the Raman amplifier in a counterpropagating direction to the PSA stage II signal.
 4. The optical systemof claim 1, wherein the input optical signal comprises one opticalchannel, and wherein the first FBG element applies the first phase shiftto the first pump wavelength.
 5. The optical system of claim 1, whereinthe input optical signal comprises a wavelength division multiplexed(WDM) optical signal, including a first optical channel, and wherein thefirst FBG element applies the first phase shift to a first idler signalthat is a conjugate of the first optical channel.
 6. The optical systemof claim 1, further comprising: a first heating element associated withthe first FBG element, wherein the first heating element is used tocontrol a local temperature of the first FBG element to control thefirst phase shift.
 7. The optical system of claim 6, further comprising:a plurality of heating elements respectively corresponding to the FBGelements, wherein each of the FBG elements applies a respective phaseshift to the PSA stage I optical signal.
 8. The optical system of claim1, further comprising: a first tension element associated with the firstFBG element, wherein the first tension element is used to control alocal strain of the first FBG element to control the first phase shift.9. The optical system of claim 8, further comprising: a plurality oftension elements respectively corresponding to the FBG elements, whereineach of the FBG elements applies a respective phase shift to the PSAstage I optical signal.
 10. A phase-sensitive amplifier, comprising: aphase-sensitive amplifier (PSA) stage I receiving an input opticalsignal, wherein the PSA stage I comprises a first non-linear opticalelement (NLE) through which the input WDM optical signal and a firstpump wavelength are passed to generate a PSA stage I optical signalcomprising the input optical signal, the first pump wavelength, and anidler signal generated using the first NLE; a fiber Bragg grating (FBG)receiving the PSA stage I optical signal, wherein the FBG includes aplurality of FBG elements, including a first FBG element configured toapply a first phase shift to at least one of the input optical signal,the first pump wavelength, and the idler signal, wherein the FBG outputsthe PSA stage I optical signal with the first phase shift as a PSA stageII optical signal, and wherein the input optical signal, the first pumpwavelength, and the idler signal are phase-matched in the PSA stage IIoptical signal; and a feedback control loop from the PSA stage II to theFBG, wherein the feedback control loop is used to spectrally align thefirst phase shift.
 11. The phase-sensitive amplifier of claim 10,further comprising: a PSA stage II receiving the PSA stage II opticalsignal, wherein the PSA stage II comprises a second NLE through whichthe PSA stage II optical signal is amplified to generate an outputoptical signal.
 12. The phase-sensitive amplifier of claim 11, whereinthe PSA stage II further comprises: a Raman amplifier; and a second pumpwavelength for transmission through the Raman amplifier in a counterpropagating direction to the PSA stage II signal.
 13. Thephase-sensitive amplifier of claim 10, wherein the input optical signalcomprises one optical channel, and wherein the first FBG element appliesthe first phase shift to the first pump wavelength.
 14. Thephase-sensitive amplifier of claim 10, wherein the input optical signalcomprises a plurality of optical channels, including a first opticalchannel, and wherein the first FBG element applies the first phase shiftto a first idler signal that is a conjugate of the first opticalchannel.
 15. The phase-sensitive amplifier of claim 10, furthercomprising: a first heating element associated with the first FBGelement, wherein the first heating element is used to control a localtemperature of the first FBG element to control the first phase shift.16. The phase-sensitive amplifier of claim 15, further comprising: aplurality of heating elements respectively corresponding to the FBGelements, wherein each of the FBG elements applies a respective phaseshift to the PSA stage I optical signal.
 17. The phase-sensitiveamplifier of claim 10, further comprising: a first tension elementassociated with the first FBG element, wherein the first tension elementis used to control a local strain of the first FBG element to controlthe first phase shift.
 18. The phase-sensitive amplifier of claim 17,further comprising: a plurality of tension elements respectivelycorresponding to the FBG elements, wherein each of the FBG elementsapplies a respective phase shift to the PSA stage I optical signal.